Serve Bellatrix preset in BN API (#3425)

## Issue Addressed

Resolves #3388
Resolves #2638

## Proposed Changes

- Return the `BellatrixPreset` on `/eth/v1/config/spec` by default.
- Allow users to opt out of this by providing `--http-spec-fork=altair` (unless there's a Bellatrix fork epoch set).
- Add the Altair constants from #2638 and make serving the constants non-optional (the `http-disable-legacy-spec` flag is deprecated).
- Modify the VC to only read the `Config` and not to log extra fields. This prevents it from having to muck around parsing the `ConfigAndPreset` fields it doesn't need.

## Additional Info

This change is backwards-compatible for the VC and the BN, but is marked as a breaking change for the removal of `--http-disable-legacy-spec`.

I tried making `Config` a `superstruct` too, but getting the automatic decoding to work was a huge pain and was going to require a lot of hacks, so I gave up in favour of keeping the default-based approach we have now.

lighthouse/tests/beacon_node.rs

@@ -11,7 +11,7 @@ use std::process::Command;
 use std::str::FromStr;
 use std::string::ToString;
 use tempfile::TempDir;
-use types::{Address, Checkpoint, Epoch, ExecutionBlockHash, Hash256, MainnetEthSpec};
+use types::{Address, Checkpoint, Epoch, ExecutionBlockHash, ForkName, Hash256, MainnetEthSpec};
 use unused_port::{unused_tcp_port, unused_udp_port};
 
 const DEFAULT_ETH1_ENDPOINT: &str = "http://localhost:8545/";
@@ -949,6 +949,21 @@ fn http_tls_flags() {
         });
 }
 
+#[test]
+fn http_spec_fork_default() {
+    CommandLineTest::new()
+        .run_with_zero_port()
+        .with_config(|config| assert_eq!(config.http_api.spec_fork_name, None));
+}
+
+#[test]
+fn http_spec_fork_override() {
+    CommandLineTest::new()
+        .flag("http-spec-fork", Some("altair"))
+        .run_with_zero_port()
+        .with_config(|config| assert_eq!(config.http_api.spec_fork_name, Some(ForkName::Altair)));
+}
+
 // Tests for Metrics flags.
 #[test]
 fn metrics_flag() {